The present invention relates to a system for rotating a pedestal with at least two coupled actuators.
Fiber optic networks have the potential for greatly increasing telecommunication bandwidths and data rates. The demand for increased capacity continues to grow, especially as more and more information is transmitted across the Internet.
One limitation of fiber optic networks as currently implemented is their inability to directly switch optically encoded packets of data from a fiber on a source network or network node to a fiber on a destination network or network node. Instead, optically encoded data are dropped from the source network fiber, converted to electrically encoded data, switched to the destination network using conventional electronic switches, converted back into optically encoded data, and injected into the destination network fiber.
Micromachined mirror arrays offer the ability to directly switch optically encoded data in devices, known as all-optical cross connect switches, from a source fiber on a source network to a destination fiber on a destination network without having to convert the data from optical to electronic and back again. For such mirror arrays to be commercially useful, they must be able to cross connect approximately 1000 input fibers with an equal number of output fibers in a compact volume. This can be achieved with mirrors that can be densely packed together and that are rotatable by relatively large angles in an arbitrary angular direction.
Recent developments in the field of microelectomechanical systems (MEMS) allow for the bulk production of microelectromechanical mirrors and mirror arrays that can be used in all-optical cross connect switches. MEMS-based mirrors and mirror arrays can be inexpensively designed and produced using conventional tools developed for the design and production of integrated circuits. Such tools include computer-aided design, photolithography, bulk and surface micromachining, wet and dry isotropic and anisotropic etching, and batch processing. In addition, deep reactive ion etching methods (DRIE) allow silicon devices to be produced having high aspect ratios (xcx9c20:1) that rival those that can be achieved using the prohibitively expensive lithography, electroplating and molding process (LIGA) which requires access to a synchrotron radiation source. (LIGA is an acronym for the German lithographic, galvanoformung und abformung).
A number of microelectromechanical mirror arrays have already been designed for use with MEMS production processes and techniques.
In U.S. patent application Ser. No. 09/779,189 of Nasiri, filed on Feb. 7, 2001, and hereby incorporated by reference in its entirety, a mirror is mounted on a support post mounted on a freely moving plate. In Nasiri, two orthogonally oriented pairs of rotatable actuators are coupled to the freely moving plate by gimbal springs. By properly coordinating each pair of actuators, the mirror can be rotated without displacement under ideal conditions.
Although the Nasiri application shows improved ability to manipulate the mirror rotation without displacement, the performance of similar configurations can be greatly improved by paying special attention to the system used for transmitting rotation from the actuators to the freely moving plate. U.S. patent application Ser. No. 10/225,081 of Starr et al, filed on Aug. 20, 2002 and hereby incorporated by reference in its entirety, discloses special gimbal springs and lever arms for coupling the actuators to a gimbaled platform.
Improvements to the performance of such systems for rotating mirrors can be realized by further reducing out-of-plane displacements of the system. These displacements, which are perpendicular to the mirror face when the mirror is in the neutral position, are also known as z-displacements, and the out-of-plane direction is characterized as the z-direction. With mirror designs similar to those of Nasiri and/or Starr, et al, to achieve large mirror rotations, the torsion springs supporting the actuators need to be relatively compliant. However, compliant torsion springs cause the mirror to be weakly supported in the z-direction. At least two problems are associated with the relatively weak support in the z-direction.
First, weak support in the z-direction can detrimentally affect mirror control. External disturbances in the z-direction cause the mirror to displace and thereby rotate the actuators about their respective axes. Because the actuator gains are typically a function of actuator rotation, the gain of the actuators varies in spite of the fact that the mirror rotation has not changed. If the mirror is controlled by closed-loop servo with mirror rotation as an error feedback, then the change in actuator rotation is unknown and results in errors.
Secondly, if electrostatic actuation is utilized, then the compliancy of the z-displacement creates an unstable snapdown mode wherein actuators on opposite sides of the mirror snap down through opposite-sense rotations. This mode occurs at a lower voltage than the well-known fundamental electrostatic snapdown mode of rotational electrostatic actuators. This lower-voltage z-direction snapdown mode limits the maximum achievable mirror angle.
The current invention couples two actuators in a manner that inhibits their ability to produce out-of-plane displacements of the system.
In a preferred embodiment, the apparatus comprises a pedestal and first and second rotatable actuators having first and second actuator rotation axes, respectively. The actuator rotation axes are substantially parallel to each other, and define an actuator plane. First and second linkage arms are attached to the first and second rotatable actuators, respectively. First and second gimbal springs connect the respective linkage arms to the pedestal. In preferred embodiments, coordinated rotation of the actuators in the same sense with respect to their axes tends to rotate the pedestal with minimal out-of-plane displacement of the centroid of the pedestal. Simultaneous opposite sense rotations of the first and second rotatable actuators tend to produce out-of-plane displacement of the centroid of the pedestal. Preferred embodiments include a coupling mechanism between the first and second rotatable actuators that inhibits the simultaneous opposite sense rotation of the first and second rotatable actuators. For the purposes herein, the coupling mechanism is a distinct mechanism. Hence the implicit coupling of the first and second rotatable actuators through their respective linkage arms and gimbal springs connected to the pedestal is not to be included in the broad meaning of coupling mechanism as used herein.
Preferred embodiments of the invention can be considered as a method for inhibiting out-of-plane motion of the pedestal. Gimbal springs and linkage arms connect the pedestal to a pair of rotatable actuators, each rotatable actuator having an actuator rotation axis. The out-of-plane motion of the pedestal is inhibited by coupling the pair of rotatable actuators such that the torque for rotating the rotatable actuators increases faster per actuator angle of rotation for actuator rotations in the opposite sense than for actuator rotations the same sense.
Additional features and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. Various embodiments of the invention do not necessarily include all of the stated features or achieve all of the stated advantages.